The preparation of wet-laid composite sheets is generally well known in the art. A composite sheet material can be of a cellulose, polymeric or other fibrous material product prepared by methods as outlined in U.S. Pat. Nos. 4,225,383; 4,189,345 and 4,187,142. The techniques for manufacturing composite can include an aqueous wet-laid method or a beater addition process for converting fibers into a sheet or paper material.
Typically, a latex is employed as a binder in the preparation of a composite sheet for two roles. The first is a wet-end role wherein the latex assists in the preparation of the composite sheet into a unitary mass. The second is an end-use role wherein the physical properties of the latex are contributed to the overall properties of the sheet.
Wet-end characteristics are important to the efficient preparation of composite sheets while end-use characteristics are important to the final properties of the composite sheet. Unfortunately, a latex which has good wet-end properties may not yield good end-use properties. For example, optimization of the wet-end properties of deposition time and drainage time will usually result in a final product having low end-use properties such as tensile strength. On the other hand, optimization of tensile strength can lead to poor drainage time and deposition time. Therefore, it would be desirable to prepare a single latex composition having both good wet-end and end-use properties for the preparation of wet-laid composite materials.
One solution is to blend various latexes especially designed for wet-end properties and end-use properties. However, this would involve the addition of at least two latexes to the aqueous slurry for preparing a composite sheet, which carries inherent problems of limited drain time and strength. Also, employing more than one latex would involve extra preparation, handling and storage.
Another solution to preparing a latex having both good wet-end and end-use properties is to copolymerize various monomers together; however, this is not always satisfactory because the wet-end properties are predominantly affected by the surface chemistry of the latex polymer particles and the softness of the latex polymer particles. Thus, the mere copolymerization of various monomers would not allow one to adjust the wet-end properties independently from the end-use properties which depend largely on the hardness of the polymers.
The subject invention is directed toward the preparation of structured latex particles having a shell/core morphology whereby the wet-end and end-use properties are optimized. Structured latex polymer particles are generally known in the art as demonstrated by U.S. Pat. Nos. 4,017,442; 4,156,669 and 4,325,856. It has now been discovered that, by careful adjustment of structured latex polymer particle parameters, a binder can be formed having good wet-end and end-use properties for composite sheets.